A Parallelogram is a four-sided figure, whose opposite sides are parallel.

The Diagonal of a parallelogram is a line drawn through it from one angle to the angle opposite.

A Square is a parallelogram, all whose sides are equal, and all the angles right angles.

A Parallelogram.

An Oblong.

An Oblong is a parallelogram whose angles are right angles, but its sides are not all equal.

A Pentagon is a five-sided figure, and a Hexagon a six-sided figure; an Octagon is an eight-sided figure.

A Circle is a figure contained by a line which is such

that any point in it is at the same distance from one point inside the circle. This one point is called the centre. The line which bounds the circle is called the

circumference, or periphery. Every straight line drawn from the circumference to the centre is called a radius of the circle. A straight line drawn from any point in the circumference, through the centre, to meet the circumference again, is called a diameter.

A diameter cuts a circle into two halves; each half is called a semicircle.

A part of the circumference is called an arc; a part of the circle cut off by a straight line is a segment of a circle.

The circumference is concave to one who stands within it, and convex to one who stands outside of it. A tangent is a straight line which touches a circle.

CHAPTER LX.

MECHANICAL POWERS.-PART I.

MANY animals are endowed with marvellous skill. Birds with their bills can frame nests, which no human art can imitate; beavers, with their teeth and paws, can cut down trees, build dams, and construct houses to dwell in; but God has given to man a wisdom which is more serviceable than the skill of all other animals. With his hand alone he can do much; but by his wisdom he can contrive means of doing infinitely more than any of the most sagacious and most skilful of the brute creation.

Man can construct tools and instruments with which to work; and this is one great difference between a man and a brute, that man can make tools.

The art by which we work with tools or instruments is called mechanical, which means the contriving art.

A Machine is a contrivance by which any force is made serviceable for a particular work; and so any tool might be properly called a machine. But the word machine is generally used of those contrivances in which there is an arrangement of several parts. There are a few simple means of applying force which are called the mechanical powers, and machines are constructed by making use of them in various ways.

1st. The Lever.-If you wish to move a large block of stone, you may take an iron bar five or six feet long. Then, laying a block of wood near the stone, place the bar so that it rests against the wooden block, with one end under the stone. You will find that by bearing your weight on the other end of the bar, you can raise a stone, which you could not stir with your hands alone. This contrivance is called a Lever. The block of wood

which serves as a rest is called a Fulcrum, which is the Latin word for Prop. The parts of the bar on either side of the fulcrum are called the Arms of the lever. The nearer the fulcrum is to the stone, the more easily will it be raised; and if the fulcrum remain in the same place, a long bar will move the stone with less effort than a shorter one. If you had a very long bar, strong enough not to bend, and could arrange the fulcrum

properly, you might in this way move enormous weights, though it would be very slowly.

We are constantly in the habit of using levers. If a big and a little boy are playing at seesaw on a plank over the trunk of a tree, the big boy places himself nearer the trunk, and the little boy farther off. In this way a small boy can raise up a great one. This plank and trunk form a Lever, the part of the trunk on which the plank rests is the Fulcrum, and the parts of the plank on either side the trunk are the Arms of the lever. A common pump is moved by a lever; the pivot or pin on which the handle turns is the fulcrum; the sucker is at the end of the short arm, and the handle is the long arm of the lever.

A steelyard is a lever. This is a bar of iron used to weigh meat, and is hung up from a point in the bar much nearer one end than the other. At the end of

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the short arm is a hook, and on the long arm there is a sliding weight. If you attach to the hook a piece of meat weighing six pounds, you will, by sliding the weight upon the long arm, find some point at which the weight will exactly balance the meat on the other side. The point from which the steelyard is hung is the fulcrum, the two parts of the bar on either side are the arms of the lever, and here a less weight supports a greater by being placed farther from the fulcrum.

A lever sometimes has only one arm. If a rod be fixed to a wall by means of a hinge, and held out

straight, with a weight hung to it, it will be found that the nearer the weight is to the hinge the easier it will be to support it. Here the hinge is the fulcrum.

Levers are not always used to raise weights. A pair of scissors is composed of two separate levers, turning round a pivot, which forms a fulcrum to each lever. A pair of tongs is also a double lever, with a hinge for a fulcrum. Here each lever has but one arm, and the result produced is to press the two ends together, so as to hold a coal between them. The hands which act upon the levers are nearer the fulcrum than the ends which hold the coal; but this does not signify-we do not want to increase our power, but to make it act at a distance.

2nd. The Wheel and Axle.-Suppose a wheel and an axle fixed in a frame, as in the following figure:

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The wheel is moved by a cord passing round it, and a weight raised by another cord wound round the axle. The larger the wheel is in proportion to the axle, the easier it is to raise the weight.

A common windlass for drawing water from a well is

an example of this machine. The bucket of water is